In general, a hydraulic excavator, which belongs to construction machinery, includes a prime mover such as an engine, a hydraulic pump driven by the prime mover, a hydraulic actuator, and a control valve for switching to the hydraulic actuator and supplying the hydraulic oil from the hydraulic pump to the hydraulic actuator, the hydraulic actuator being for driving, for example, a boom, an arm, a bucket, or a swing structure by use of a hydraulic oil discharged from the hydraulic pump. A technology proposed for such construction machinery retrieves the potential energy of the boom falling under its own weight and the kinetic energy of inertia of the swing structure and makes effective use of the recovered energy in order to reduce the motive power of a motive power source for the purpose of reducing the fuel consumption of the whole construction machinery.
A hydraulic oil energy recovery/regeneration device disclosed, for instance, in Patent Document 1 includes a hydraulic actuator, a recovery device, an energy storage device, and regeneration device. The hydraulic actuator is driven when hydraulic oil discharged from a hydraulic actuator drive hydraulic pump is supplied. The recovery device recovers returning hydraulic oil that flows out of the hydraulic actuator. The energy storage device converts the energy of the recovered returning hydraulic oil to a predetermined energy and stores the resulting energy. The regeneration device uses the energy stored in the energy storage device to supplement the energy for driving the hydraulic actuator drive hydraulic pump. The energy storage device includes a recovery hydraulic motor, a generator, and a battery. The recovery hydraulic motor is driven when the returning hydraulic oil flowing out of the hydraulic actuator flows into the recovery hydraulic motor. The generator generates electric energy when the driving force of the recovery hydraulic motor is input to the generator. The battery stores the electric energy generated by the generator. The regeneration device includes a regeneration device that uses the electric energy stored in the battery to supplement the energy for driving the hydraulic actuator drive hydraulic pump.